Referring to FIGS. 11 and 12, a general configuration of a common mode choke coil 31 will be described.
As illustrated in FIG. 11, the common mode choke coil 31 includes a core 32, and first and second wires 33 and 34 each forming an inductor. The core 32 is made of an electrical insulating material, more specifically, a material such as alumina as an example of a dielectric, a Ni—Zi-based ferrite as an example of a magnetic material, or resin. The core 32 has a substantially rectangular cross-sectional shape as a whole. The wires 33 and 34 are formed by, for example, a copper wire with an insulating coating.
The core 32 has a winding core part 35, and first and second flange parts 36 and 37 provided at opposite end portions of the winding core part 35. The first and second wires 33 and 34 are wound around the winding core part 35 in a substantially helical manner with a substantially equal number of turns while running parallel to each other from a first end portion 38 where the first flange part 36 is located toward a second end portion 39 where the second flange part 37 is located.
First and second terminal electrodes 41 and 42 are provided in the first flange part 36, and third and fourth terminal electrodes 43 and 44 are provided in the second flange part 37. The terminal electrodes 41 to 44 are formed by a method such as baking of an electrically conductive paste or plating of an electrically conductive metal. As can be appreciated from the locations of the terminal electrodes 41 to 44, FIG. 11 depicts the common mode choke coil 31 in such a position that its mounting surface, which is orientated toward the mount board, faces upward.
Opposite end portions of the first wire 33 are connected to the first and third terminal electrodes 41 and 43, and opposite end portions of the second wire 34 are connected to the second and fourth terminal electrodes 42 and 44. These connections are made by, for example, thermo-compression bonding.
The common mode choke coil 31 further includes a top plate 45. Like the core 32, the top plate 45 is made of an electrical insulating material, more specifically, a material such as alumina as an example of a dielectric, a Ni—Zi-based ferrite as an example of a magnetic material, or resin. If the core 32 and the top plate 45 are each made of a magnetic material, when the top plate 45 is disposed so as to connect the first and second flange parts 36 and 37 with each other, the core 32 forms a closed magnetic circuit in cooperation with the top plate 45.
The common mode choke coil 31 configured as described above gives an equivalent circuit as illustrated in FIG. 12. In FIG. 12, elements corresponding to those illustrated in FIG. 11 are denoted by the same reference signs.
Referring to FIG. 12, the common mode choke coil 31 includes a first inductor 46, and a second inductor 47. The first inductor 46 is formed by the first wire 33 connected between the first and third terminal electrodes 41 and 43. The second inductor 47 is formed by the second wire 34 connected between the second and fourth terminal electrodes 42 and 44. The first and second inductors 46 and 47 are magnetically coupled to each other.
Although not clearly illustrated in FIG. 11, the first wire 33 is wound so as to form a first layer that contacts the peripheral surface of the winding core part 35, and the second wire 34 is wound so as to form a second layer on the outer side of the first layer, with a part of the second wire 34 fitting in the recess defined between adjacent turns of the first wire 33.
A problem often encountered by the common mode choke coil 31 mentioned above with increased frequency of signals input to the common mode choke coil 31 is the increased mode conversion characteristics, which represent the proportion of input differential signal components that are converted into and output as common mode noise. For example, Japanese Unexamined Patent Application Publication No. 2014-120730 cites imbalance in stray capacitance (distributed capacitance) generated between different turns of the first and second wires 33 and 34 as the cause of this problem.
Accordingly, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2014-120730 employs, for example, the manner of winding the wires 33 and 34 as illustrated in FIG. 13.
In FIG. 13, the cross-sections representing the first wire 33 are shaded to clearly distinguish the first wire 33 from the second wire 34. Further, the ordinal numbers of turns “1” to “12” as counted from the first end portion 38 of the winding core part 35 are written within the respective cross-sections of the first and second wires 33 and 34 illustrated in FIG. 13.
In FIG. 13, among various portions of the first and second wires 33 and 34 wound around the winding core part 35, the portions located forward of the winding core part 35 and the portions hidden behind the winding core part 35 are schematically indicated respectively by solid and broken lines. It is to be noted that FIG. 13 does not depict all of the portions of the wires 33 and 34 located forward of the winding core part 35 and hidden behind the winding core part 35.
Referring to FIG. 13, the winding core part 35 has a first winding region A, a switching region C, and a second winding region B in this order along the axis of the winding core part 35.
(1) In the first winding region A, the respective same-numbered turns of the first and second wires 33 and 34 lie adjacent to each other with each turn of the first wire 33 being located closer to the first end portion 38 than the corresponding same-numbered turn of the second wire 34.
(2) In the second winding region B, the respective same-numbered turns of the first and second wires 33 and 34 lie adjacent to each other with each turn of the first wire 33 being located closer to the second end portion 39 than the corresponding same-numbered turn of the second wire 34.
(3) In the switching region C located between the first winding region A and the second winding region B, the first wire and the second wire 34 cross each other such that the relative positions of the turns of the first wire 33 and the turns of the second wire 34 are switched.
In addressing the problem of increased mode conversion, the technique described in Japanese Unexamined Patent Application Publication No. 2014-120730 makes the winding structure of the wires 33 and 34 in the first winding region A and the winding structure of the wires 33 and 34 in the second winding region B symmetric about a centerline C1 of the switching region C in order to balance out stray capacitances (distributed capacitances) generated between different turns of the first and second wires 33 and 34. In other words, the number of turns of each of the wires 33 and 34 in the first winding region A, and the number of turns of each of the wires 33 and 34 in the second winding region B are made substantially equal to each other.
According to Japanese Unexamined Patent Application Publication No. 2014-120730, the winding structure of the wires 33 and 34 is made symmetric as mentioned above so that the distributed capacitance in the first winding region A and the distributed capacitance in the second winding region B are respectively generated in parallel to the first and second inductors 46 and 47 (see FIG. 12). This causes the resonance point of the LC circuit formed by the first wire 33 and the resonance point of the LC circuit formed by the second wire 34 to both change, but the balance between the two resonance points remains unchanged, thus making it possible to reduce mode conversion.